Amphibians (class Amphibia) are a taxon of animals that include
all tetrapods (four-legged vertebrates) that do not have amniotic
eggs. Amphibians are ectotherms (meaning their body temperature
adjusts according to their environment), and generally spend part
of their time on land, but most do not have the adaptations to an
entirely terrestrial life. There are 6,022 described, living species
of amphibians. The study of amphibians and reptiles is known as
herpetology.

Amphibians developed with the characteristics of pharyngeal slits/gills,
a dorsal nerve cord, a notochord, and a post-anal tail at different
stages of their life.

Throughout their history, amphibians have ranged in size from large
forms, such as the 5 foot (150cm) long Eryops of the Permian period,
and the Giant Japanese and Chinese Salamanders of today, down to
the tiny Brachycephalus didactylus (Brazilian Gold Frog) and Eleutherodactylus
iberia (leptodactylid frog) from Cuba, with a total length of 9.6-9.8
millimeters (0.4 inches). Amphibians have mastered almost every
climate on earth from the hottest deserts to the frozen arctic.

For the purpose of reproduction most amphibians are bound to fresh
water. A few tolerate brackish water, but there are no true sea
water amphibians. Several hundred frog species in adaptive radiations,
however, do not need any water whatsoever. They reproduce via direct
development, an ecological and evolutionary adaptation that has
allowed them to be completely independent from free-standing water.

Almost all of these frogs live in wet tropical rainforests and
their eggs hatch directly into miniature versions of the adult,
bypassing the tadpole stage entirely. Several species have also
adapted to arid and semi-arid environments, but most of them still
need water to lay their eggs. Symbiosis with single celled algae
that lives in the jelly-like layer of the eggs has evolved several
times. The larvae (tadpoles or polliwogs) breathe with exterior
gills. After hatching, they start to transform gradually into the
adult's appearance. This process is called metamorphosis. Typically,
the animals then leave the water and become terrestrial adults,
but there are many interesting exceptions to this general way of
reproduction.

The most obvious part of the amphibian metamorphosis is the formation
of four legs in order to support the body on land. But there are
several other changes:

The gills are replaced by other respiratory organs, i.e. lungs

The skin changes and develops glands to avoid dehydration

The eyes get eyelids and adapt to vision outside the water

An eardrum is developed to lock the middle ear

In frogs and toads, the tail disappears

Dramatic declines in amphibian populations, including population
crashes and mass localized extinction, have been noted in the past
two decades from locations all over the world, and amphibian declines
are thus perceived as one of the most critical threats to global
biodiversity. A number of causes are believed to be involved, including
habitat destruction and modification, over-exploitation, pollution,
introduced species, climate change, and disease. However, many of
the causes of amphibian declines are still poorly understood, and
amphibian declines are currently a topic of much ongoing research.

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